For many years it was the universal practice to construct roofs with a waterproof layer or membrane on the outer surface thereof. Such roofing is still used in many installations, but has many disadvantages. The waterproof membrane, which may be bituminous built-up sheet or which may be a single sheet of waterproof material, is exposed to extreme temperature variations, as much as 210 degrees F., to ultraviolet radiation, and to physical abrasion, all of which have a deleterous effect on the life of the roofing.
It has been common practice for a great many years to provide insulation in roof construction, and when insulation is provided below the waterproof membrane, in the roofing system outlined above, it is often necessary to provide a second waterproof membrane below the insulation to prevent moisture from within the building from condensing in the insulation and inhibiting or destroying its insulating qualities.
An alternative upside-down roofing construction is known in which the insulation is applied over the waterproof membrane, see for example U.S. Pat. Nos. 3,411,256 and 3,763,614. In this alternative roof construction the waterproof membrane, which may be a built-up membrane or a single waterproof layer such as of elastomeric, plastomeric, liquid applied or modified bitumen is applied directly to the surface of the roof. Blocks of foam plastic insulation are then applied over the waterproof membrane. Polystyrene plastic resin foam is a superior product for such use, and STYROFOAM brand plastic foam made by Dow Chemical Company is a preferred example. It is a tough, closed cell rigid plastic foam having excellent moisture resistance and high compressive strength.
The polystyrene foam insulation placed over the waterproofing membrane rather than under the membrane protects the membrane from the effects of thermal cycling, temperature extremes, and physical abuse, thus reducing maintenance costs and prolonging the life of the entire roofing system. It has been found that the membrane so protected remains at stable temperatures below 100 degrees F. even in hot summer weather. In fact, under normal conditions, the temperature of the membrane will remain within 15-20 degrees F. of the building's inside temperature.
Typically, a polymeric fabric is installed over the foam to stablize the system, and crushed stone or gravel ballast is applied to counteract the buoyancy of the insulation boards, to provide flammability resistance to the roof surface, and to shield the foam and fabric from ultraviolet radiation. As an alternative, paving blocks may be used in place of stone, particularly if traffic is to be expected on the roof.
When traffic is expected, as in the construction of a plaza deck, pedestals or stone are provided to space the paving blocks above the top of the foam insulation to permit adequate air circulation for drying of the roofing system on warm, dry days. It will be appreciated that a base roof or deck of substantial strength must be provided to support the weight of such a roofing system.